The demand for ever greater efficiency gains in gas turbine engines has lead to the demand for ultra-fine (low surface roughness) airfoils that have a surface roughness Ra in the region of 1 to 5 micro-inches. NASA has demonstrated that an industry-standard surface finished compressor rotor blade ultrapolished, also known by super finishing, super polishing, ultra finishing and high precision surface finishing, to a 5 micro-inch finish can produce an increase in engine efficiency of approximately 0.5%, William B. Roberts et al, The Effect of Ultrapolish on a Transonic Axial Rotor, ASME Turbo Expo 2005 International Gas Turbine and Aeroengine Congress Reno Nevada, Jun. 6 to 9, 2005.
It is widely known that media finishing processes, such as those recipes that are commonly provided with media finishing equipment sold by the Rosler, Sweco, Giant, Royson, etc., are able to polish most metal surfaces to achieve surface roughness Ra measurements in the region of 7 to 25 micro-inches. The media finishing process typically comprises a tub style, batch bowl, or a continuous flow-through vibratory finisher filled with hard ceramic media stones of various shapes, abrasive content and sizes, that is vibrated with an electric motor that spins an eccentric weight. Hard ceramic media is loaded into the bowl and the act of vibrating the bowl causes that media to flow in a directional manner and circulate around the bowl. Water and burnishing compounds are typically added to the bowl to assist in the polishing, and sometimes a paste or powder may also be added to accelerate the process. The articles that are to be polished are added to the bowl so that they flow around with the media. The parts can also be fixed in a stationary position in the bowl, but this is not typical. An example of a suitable polishing machine is shown in U.S. Pat. No. 6,261,154, which is incorporated herein by reference.
High energy finishing processes such as high energy tumbling or centrifugal finishing and drag-finishing are able to achieve lower surface finish conditions. However, the high energy nature of these processes can result in the loss of material at sharp edges which may harm the dimensions of the part.
When it comes to polishing close-toleranced parts such as gas turbine engine airfoils, the polishing process can be very aggressive on sharp radius edges and corners such as the leading and trailing edges of the airfoils and blade tip corners. Changes in the dimensions of the leading and trailing edges and blade tip corners can have a profoundly detrimental effect on the mechanical properties and aerodynamic efficiency of the airfoils. Thus, a process for super-polishing close-toleranced airfoils must be able to preserve the dimensions of these areas and possibly others.